Catalytic carbon formation: clarifying the alternative kinetic routes and defining a kinetic linearity for sustained growth concept

Understanding stable carbon formation mechanisms requires isothermal kinetic studies. Distinguishing preliminary solid-state changes from the prevailing steady-state (sustained) carbon formation route is essential. The three alternative kinetic routes for carbon formation are clarified: dual catalyst route, gas phase pyrolysis and hybrid route—a combination of the two previous ones. When kinetic linearity is observed in a reaction involving catalytic formation or gasification of a solid, this is evidence that only a steady-state carbon diffusion process is operating obeying Fick’s 1st law in a stable geometry. Fick’s 2nd law rules in the initial transition stage, involving nucleation and solid-state reaction processes. Catalyst duality is discussed, considering the roles played by the two distinct surface regions between which the bulk carbon flux is maintained during steady-state reaction.